Oxide high-temperature superconductor and its production method

ABSTRACT

Disclosed is an oxide high temperature superconductor having a crystalline substrate of low dielectric constant formed thereon with a thin film of the oxide high temperature superconductor that is high in crystallographic integrity and excels in crystallographic orientation as well as a method of making such an oxide high temperature superconductor. In fabricating an oxide high temperature superconductor containing Ba as a constituent element and having such a substrate formed thereon with a thin film of the oxide high temperature superconductor, a first buffer layer composed of CeO 3  is formed on a sapphire R (1, −1, 0, 2) face substrate for reducing lattice mismatch between the sapphire R (1, −1, 0, 2) face substrate and the oxide high temperature superconductor thin film, and a second buffer layer composed of such an oxide high temperature superconductor but in which Ba is substituted with Sr is formed on the first buffer layer made of CeO 3  to allow the oxide high temperature superconductor thin film to be formed on the second buffer layer. Thus, if the first buffer layer for reducing the lattice mismatch between the sapphire R (1, −1, 0, 2) face substrate and the oxide high temperature superconductor thin film is liable to an interfacial reaction with Ba from the oxide high temperature superconductor thin film, the second buffer layer prevents the interfacial reaction, thereby permitting the epitaxial growth of an oxide high temperature superconductor thin film that excels on both crystallographic integrity and crystallographic orientation.

TECHNICAL FIELD

[0001] The present invention relates to an oxide high temperaturesuperconductor that excels in high-frequency properties and a method ofmaking such an oxide high temperature superconductor.

BACKGROUND ART

[0002] Of oxide high temperature superconductors, a Cu familysuperconductor thin film (see Kotai Butsuri (Solid State Physics), Vol.35, No. 5, 2000) excels in superconducting properties, and research anddevelopment efforts have diversely been devoted to putting it topractical use. The excellent superconducting characteristics which a Cufamily superconductor thin film possesses include the feature that itexcels in high-frequency properties as mentioned in the literaturereferred to above. In order to fabricate a superconductor thin film thatcan be applied to a high-frequency device such as a microwave device, ofimportance is not only the high-frequency properties of thesuperconductor thin film itself but also those of a substrate on whichthe superconductor thin film is to be epitaxially grown.

[0003] Fabricating a Cu family oxide high temperature superconductorthat excels in superconducting properties requires the superconductorthin film to be excellent in both crystallographic integrity andcrystallographic orientation.

[0004] For a conventional Cu family superconductor thin film, use hasbeen made of a SrTiO₃ substrate that is small in lattice mismatch withsuch a superconductor thin film and as a result is capable of growingthereon such a superconductor thin film that is high in crystallographicintegrity and excellent in crystallographic orientation. However, sinceit is large in dielectric constant (specific dielectric constant=about300), SrTiO₃ is unsuitable to form a substrate for growing asuperconductor thin film thereon for particular use in a high frequencydevice.

[0005] Thus, in order to be applicable to a high frequency device, therehas been sought an oxide high temperature superconductor in which anoxide high temperature superconductor thin film that is high incrystallographic integrity and excellent in crystallographic orientationis formed on a substrate that is low in dielectric constant, and amethod whereby such an oxide high temperature superconductor thin filmcan be simply epitaxially grown on such a substrate.

DISCLOSURE OF THE INVENTION

[0006] It is accordingly a first object of the present invention toprovide an oxide high temperature superconductor in which an oxide hightemperature superconductor thin film that is high in crystallographicintegrity and excellent in crystallographic orientation is formed on asubstrate that is low in dielectric constant.

[0007] A second object of the present invention is to provide a methodof making an oxide high temperature superconductor thin film that ishigh in crystallographic integrity and excellent in crystallographicorientation is formed on a substrate that is low in dielectric constant.

[0008] In order to achieve the first object mentioned above, there isprovided in accordance with the present invention as set forth in claim1, an oxide high temperature superconductor containing Ba as aconstituent element thereof and having a thin film thereof formed on acrystalline substrate, characterized in that it comprises: a firstbuffer layer for reducing a lattice mismatch between the saidcrystataline substrate and the said oxide high temperaturesuperconductor thin film, and a second buffer layer formed on the saidfirst buffer layer and made of an oxide high temperature superconductorthat is identical or similar in crystallographic structure and latticeconstant to the said oxide high temperature superconductor but in whichBa is substituted with Sr, interposed between the said crystallinesubstrate and the said oxide high temperature superconductor thin film.According to this makeup, Sr in the second buffer layer prevents thediffusion of Ba, and even if the first buffer layer is composed of amaterial that is liable to cause an interfacial reaction with Ba, thesecond buffer layer can prevent the interfacial reaction. Therefore, therange of choice of materials usable to form the first buffer layer isexpanded to allow selecting an optimum material for reducing the latticemismatch to a minimum. Also, since the second buffer layer is a thinfilm made of an oxide high temperature superconductor that is identicalor similar in crystallographic structure and lattice constant to theoxide high temperature superconductor but in which merely Ba issubstituted with Sr, an ultimate thin film of the oxide high temperaturesuperconductor has excellent lattice match with the second buffer layerand is thus allowed to form with excellent crystallographic orientationin both film-thickness and in-plane directions. Consequently, an oxidehigh temperature superconductor prepared according to the presentinvention comes to exhibit excellent superconducting propertiesregardless of the kind of a substrate material used.

[0009] The present invention also provides as set forth in claim 2, anoxide high temperature superconductor containing Ba as a constituentelement thereof and having a thin film thereof formed on a crystallinesubstrate, characterized in that it comprises a first buffer layer forreducing a lattice mismatch between the said crystalline substrate andthe said oxide high temperature superconductor thin film, and a secondbuffer layer formed on the said buffer layer and made of an oxide hightemperature superconductor not containing Ba but containing Sr which issimilar in crystallographic structure and lattice constant to the saidoxide high temperature superconductor, or a Sr oxide which is similar incrystallographic structure and lattice constant to the said oxide hightemperature superconductor, these two buffer layers interposed betweenthe said crystalline substrate and the said oxide high temperaturesuperconductor thin film. According to this makeup, Sr in the secondbuffer layer prevents the diffusion of Ba, and even if the first bufferlayer is composed of a material that is liable to cause an interfacialreaction with Ba, the second buffer layer can prevent the interfacialreaction. Therefore, the range of choice of materials usable to form thefirst buffer layer for reducing a lattice mismatch between an oxide hightemperature superconductor and a substrate is expanded to allowselecting an optimum material for reducing the lattice mismatch to aminimum. Also, since the second buffer layer is well lattice-matchedwith the oxide high temperature superconductor, an ultimate thin film ofthe oxide high temperature superconductor is allowed to form withexcellent crystallographic orientation in both film thickness andin-plane directions. Consequently, an oxide high temperaturesuperconductor prepared according to the present invention comes toexhibit excellent superconducting properties regardless of the kind of asubstrate material. The present invention also provides as set forth inclaim 3, an oxide high temperature superconductor containing Ba as aconstituent element thereof and having a thin film thereof formed on acrystalline substrate, characterized in that it comprises a buffer layerinterposed between the said crystalline substrate and the said oxidehigh temperature superconductor thin film for reducing a latticemismatch between them, wherein the said buffer layer reducing thelattice mismatch is made of an oxide high temperature superconductorthat is identical to the said oxide high temperature superconductorexcept that Ba is substituted with Sr therein, or an oxide hightemperature superconductor that is identical to the said oxide hightemperature superconductor except not containing Ba but containing Sr,or a Sr oxide that reduces the lattice mismatch. According to thismakeup, the buffer layer in which Sr prevents the diffusion of Ba allowsusing a substrate material that is liable to reacting with Ba and thusexpands the choice of materials usable to form the substrate. Also,since the buffer layer is made of a material having a crystallographicstructure and a lattice constant requited to reduce lattice mismatchbetween the oxide high temperature superconductor and the substrate, anultimate thin film of the oxide high temperature superconductor isallowed to form with excellent crystallographic orientation in both filmthickness and in-plane directions. Consequently, an oxide hightemperature superconductor prepared according to the present inventioncomes to exhibit excellent superconducting properties regardless of thekind of a substrate material used.

[0010] Here, the said oxide high temperature superconductor containingBa as a constituent element thereof as set forth in any one of claims 1to 3 can consist of a composition expressed by

composition formula:Cu_(1-x)M_(x)(B_(1-y)Sr_(y))₂(Ca_(1-z)L_(z))_(n−1)(Cu_(1-q)Q_(q))_(n)O_(2n+4-w)or

composition formula:(Cu_(1-x)M_(x))₂(Ba_(1-y)Sr_(y))₂(Ca_(1-z)L_(z))_(n−1)(Cu_(1-q)Q_(q))_(n)O_(2n+4-w)

[0011] where:

[0012] M represents Tl, Hg, Bi, Pb, In, Ga, Al, B, C, Si, Sn, Ag, Au, S,N, P, Mo, Re, Os, Cr, Ti, V, Fe, one element in the lanthanide series,or one or more alkali metal elements,

[0013] L represents Mg, Y, or one or more elements in the lanthanideseries.

[0014] Q represents either or both of Mg and Zn, and

[0015] 0≦x≦1,0≦y≦1, 0≦z≦1, 0≦q≦0.1, 0≦w≦4, and 2≦n≦5.

[0016] According to this makeup, an oxide high temperaturesuperconductor composed as specified above which excels in bothcrystallographic integrity and crystallographic orientation is allowedto form on a substrate of low dielectric constant. It should be notedhere that such oxide superconductors of composition as mentioned aboveinclude so-called YBCO type, Y(Ln)-[123] type and Hg type oxide hightemperature superconductors.

[0017] (Deleted)

[0018] The said crystalline substrate is preferably a sapphire substratehaving a sapphire R face (1, −1, 0, 2). Also, the first buffer layermentioned in claim 1 or claim 2 may be a CeO₂ layer, and then the secondbuffer layer may have a composition expressed by composition formula:

Cu_(1-x)M_(x)Sr₂CaCu₂O_(8-w) where

[0019] M represents Tl, Hg, Bi, Pb, In, Ga, Al, B, C, Si, Sn, Ag, Au, S,N, P, Mo, Re, Os, Cr, Ti, V, Fe, an element in the lanthanide series orone or more alkali metal elements and where

[0020] x and w are represented by 0≦x≦1 and 0≦w≦4, respectively.According to this makeup, an oxide high temperature superconductor thatexcels in both crystallographic integrity and crystallographicorientation can be obtained by virtue of the fact that the second bufferlayer composed of Cu_(1-x)M_(x)Sr₂CaCu₂O_(8-w) prevents the interfacialreaction between Ce in the first buffer layer composed of CeO₂ on thesapphire R face (1, −1, 0, 2) substrate of a low dielectric constant (aslow as about 10) and Ba in the oxide high temperature superconductorthin film being epitaxially grown.

[0021] In an oxide high temperature superconductor as set forth in claim1 or claim 2, the said oxide high temperature superconductor thin filmcontaining Ba as a constituent element may be epitaxially grown wherebyan amorphous phase composed to form this oxide high temperaturesuperconductor is deposited on the said second buffer layer and thedeposited amorphous phase is heat-treated in an oxygen atmosphere at apressure of 1.0 to 10 atm in the presence of Ag₂O or AgO, oralternatively in the presence of Tl.

[0022] In an oxide high temperature superconductor as set forth in claim3, the said oxide high temperature superconductor thin film containingBa as a constituent element may be epitaxially grown whereby anamorphous phase composed to form this oxide high temperaturesuperconductor is deposited on the said buffer layer and the depositedamorphous phase is heat-treated in an oxygen atmosphere at a pressure of1.0 to 10 atm in the presence of Ag₂O or AgO, or alternatively in thepresence of Tl.

[0023] The present invention also provides as set forth in claim 13, athin film of an oxide containing Ba as a constituent element thereofformed on a sapphire R (1, −1, 0, 2) face substrate, the oxidecontaining Ba being one selected from the group that consists of anoxide magnetic material, an oxide dielectric and an oxide conductor,characterized in that the said thin film has a laminated structureformed by: forming a first buffer layer made of a CeO₂ thin film on thesaid sapphire substrate, forming on the said first buffer layer a secondbuffer layer made of a thin film of the said oxide in which Ba issubstituted with Sr, and forming the said oxide on the said secondbuffer layer. According to this makeup, it can be possible to provide anoxide thin film that excels in properties, which contains Ba as aconstituent element and is selected one from the group that consists ofan oxide magnetic material, an oxide dielectric and an oxide conductor,by virtue of the fact that a thin film of an oxide containing Ba can beformed on the CeO₂ buffer layer on the sapphire substrate withoutreacting with Ce in the CeO₂ buffer layer.

[0024] In order to achieve the second object mentioned above there isalso provided in accordance with the present invention as set forth inclaim 14, a method of making a thin film of an oxide high temperaturesuperconductor containing Ba as a constituent element thereof byepitaxially growing the oxide high temperature superconductor thin filmon a crystalline substrate, characterized in that it comprises the stepsof: forming on the said crystalline substrate a first buffer layeradapted to reduce a lattice mismatch between the said crystallinesubstrate and the said oxide high temperature superconductor thin film;forming on the said first buffer layer a second buffer layer made of anoxide high temperature superconductor that is identical or similar incrystallographic structure and lattice constant to the said oxide hightemperature superconductor but in which Ba is substituted with Sr, anoxide high temperature superconductor not containing Ba but containingSr which is similar in crystallographic structure and lattice constantto the said oxide high temperature superconductor, or a Sr oxide whichis similar in crystallographic structure and lattice constant to thesaid oxide high temperature superconductor, and epitaxially growing thesaid oxide high temperature superconductor thin film on the said secondbuffer layer. According to this method makeup, a thin film of an oxidehigh temperature superconductor that excels in both crystallographicintegrity and crystallographic orientation can be epitaxially grown byvirtue of the fact that even if the first buffer layer for reducing thelattice mismatch between the oxide high temperature superconductor thinfilm and the substrate is composed of a material containing a substancethat is liable to interfacially reacting with Ba from the oxide hightemperature superconductor thin film, the second buffer layer preventsthe interfacial reaction.

[0025] In order to achieve the second object mentioned above there isalso provided in accordance with the present invention as set forth inclaim 15, a method of making a thin film of an oxide high temperaturesuperconductor containing Ba as a constituent element thereof byepitaxially growing the oxide high temperature superconductor thin filmon a crystalline substrate, characterized in that it comprises the stepsof: forming on the said crystalline substrate a buffer layer adapted toreduce a lattice mismatch between the said crystalline substrate and thesaid oxide high temperature superconductor thin film, wherein the saidbuffer layer reducing the lattice mismatch is made of an oxide hightemperature superconductor that is identical to the said oxide hightemperature superconductor except that Ba is substituted with Srtherein, or an oxide high temperature superconductor that is identicalto the said oxide high temperature superconductor except not containingBa but containing Sr, or a Sr oxide that reduces the lattice mismatch;and epitaxially growing the said oxide high temperature superconductorthin film on the said buffer layer. According to this method makeup, thebuffer layer in which Sr prevents the diffusion of Ba allows using asubstrate material that is liable to reacting with Ba and thus expandsthe choice of materials usable to form the substrate. Also, since thebuffer layer is made of a material having a crystallographic structureand a lattice constant requited to reduce lattice mismatch between theoxide high temperature superconductor and the substrate, an ultimatethin film of the oxide high temperature superconductor is allowed toform with excellent crystallographic orientation in both film thicknessand in-plane directions. Consequently, an oxide high temperaturesuperconductor prepared according to the present invention comes toexhibit excellent superconducting properties regardless of the kind of asubstrate material used.

[0026] Here, the said oxide high temperature superconductor containingBa as a constituent element thereof as set forth in claim 14 or claim 15can consist of a composition expressed by

composition formula:Cu_(1-x)M_(x)(Ba_(1-y)Sr_(y))₂(Ca_(1-z)L_(z))_(n−1)(Cu_(1-q)Q_(q))_(n)O_(2n+4-w)or

composition formula:(Cu_(1-x)M_(x))₂(Ba_(1-y)Sr_(y))₂(Ca_(1-z)L_(Z))_(n−1)(Cu_(1-q)Q_(q))_(n)O_(2n+4-w)

[0027] where:

[0028] M represents Tl, Hg, Bi, Pb, In, Ga, Al, B, C, Si, Sn, Ag, Au, S,N, P, Mo, Re, Os, Cr, Ti, V, Fe, one element in the lanthanide series,or one or more alkali metal elements,

[0029] L represents Mg, Y, or one or more elements in the lanthanideseries.

[0030] Q represents either or both of Mg and Zn, and

[0031] 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦q≦0.1, 0≦w≦4, and 2≦n≦5.

[0032] According to this makeup, an oxide high temperaturesuperconductor composed as specified above which excels in bothcrystallographic integrity and crystallographic orientation is allowedto epitaxially grow on a substrate of low dielectric constant. It shouldbe noted here that such oxide superconductors of composition asmentioned above include so-called YBCO type, Y(Ln)-[123] type and Hgtype oxide high temperature superconductors.

[0033] According to this method makeup, the buffer layer can be obtainedeasily, that is well lattice-matched with the said oxide hightemperature superconductor and at the same time acts as a barrier to thediffusion of Ba of the said oxide high temperature superconductor thinfilm being epitaxially grown, therefore, an oxide high temperaturesuperconductor thin film that excels in both crystallographic integrityand crystallographic orientation can be easily epitaxially grown.

[0034] The said crystalline substrate is preferably a sapphire substratehaving a sapphire R face (1, −1, 0, 2). Also, the first buffer layermentioned in claim 1 may be a CeO₂ layer, and then the second bufferlayer may have a composition expressed by composition formula:

Cu_(1-x)M_(x)Sr₂CaCu₂O_(8-w) where

[0035] M represents Tl, Hg, Bi, Pb, In, Ga, Al, B, C, Si, Sn, Ag, Au, S,N, P, Mo, Re, Os, Cr, Ti, V, Fe, an element in the lanthanide series orone or more alkali metal elements and where

[0036] x and w are represented by 0≦x≦1 and 0≦w≦4, respectively.According to this method makeup, an oxide high temperaturesuperconductor that excels in both crystallographic integrity andcrystallographic orientation can be epitaxially grown by virtue of thefact that the second buffer layer composed ofCu_(1-x)M_(x)Sr₂CaCuO_(8-w) prevents the interfacial reaction between Cein the first buffer layer composed of CeO₂ on the sapphire R face (1,−1, 0, 2) substrate of a low dielectric constant (as low as about 10)and Ba in the oxide high temperature superconductor thin film beingepitaxially grown.

[0037] In a method as set forth in claim 14, the said oxide hightemperature superconductor thin film containing Ba as a constituentelement thereof, may be epitaxially grown whereby an amorphous phasecomposed to form this oxide high temperature superconductor is depositedon the said second buffer layer and the deposited amorphous phase isheat-treated in an oxygen atmosphere at a pressure of 1.0 to 10 atm inthe presence of Ag₂O or AgO, or alternatively in the presence of Tl.

[0038] In a method as set forth in claim 15, the said oxide hightemperature superconductor thin film containing Ba as a constituentelement thereof, may be epitaxially grown whereby an amorphous phasecomposed to form this oxide high temperature superconductor is depositedon the said buffer layer and the deposited amorphous phase isheat-treated in an oxygen atmosphere at a pressure of 1.0 to 10 atm inthe presence of Ag₂O or AgO, or alternatively in the presence of Tl.

[0039] The present invention also provides as set forth in claim 24 amethod of making an oxide thin film by epitaxially growing on a sapphireR face (1, −1, 0, 2) substrate an oxide containing Ba as a constituentelement thereof and selected from the group that consists of an oxidemagnetic material, an oxide dielectric and an oxide conductor,characterized in that the method comprises the steps of:

[0040] forming a first buffer layer made of a CeO₂ thin film on the saidsapphire substrate, forming on the said first buffer layer a secondbuffer layer made of a thin film of the said oxide in which Ba issubstituted with Sr, and epitaxially growing the said oxide on the saidsecond buffer layer. According to this method makeup, an oxide thin filmthat excels in properties can be epitaxially grown by virtue of the factthat a thin film of an oxide containing Ba as a constituent elementthereof and selected from the group that consists of an oxide magneticmaterial, an oxide dielectric and an oxide conductor can be formed onthe CeO₂ buffer layer on the sapphire substrate without reacting with Cein the CeO₂ buffer layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The present invention will better be understood from thefollowing detailed description and the drawings attached hereto showingcertain illustrative forms of implementation of the present invention.In this connection, it should be noted that such forms of implementationillustrated in the accompanying drawings hereof are intended in no wayto limit the present invention but to facilitate an explanation andunderstanding thereof. In the drawings:

[0042]FIG. 1 shows an AFM (Atomic Force Microscopic) image of a surfaceof a CeO₂ (100) layer grown on a sapphire R (1, −1, 0.2) face substrate;

[0043]FIG. 2 is a diagram showing a diffraction pattern by XRD (X-raydiffractometer) of an oxide high temperature superconductor fabricatedin accordance with the present invention;

[0044]FIG. 3 is a diagram showing results of measurement by XRD of anin-plane orientation of the oxide high temperature superconductorfabricated in accordance with the present invention; and

[0045]FIG. 4 is a diagram showing a diffraction pattern by XRDindicating that Sr in a second buffer layer does not react with Ce in afirst buffer layer, which layers are made in accordance with the presentinvention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0046] Hereinafter, the present invention will be described in detailwith reference to certain suitable forms of implementation thereofillustrated in the drawing figures.

[0047] At the outset, mention is made of an oxide high temperaturesuperconductor fabricating method of the present invention as regards aspecific form of implementation thereof.

[0048] Of superconductors having a superconducting transitiontemperature of not less than 77 K, a Cu family oxide high temperaturesuperconductor such as CuTl-[1223] or CuTl-[1234] (see Kotai Butsuri[Solid State Physics], Vol. 35, No.5, 2000), namely having a compositionexpressed by

composition formula: Cu_(1-x)Tl_(x)Ba₂Ca₂Cu₃O_(8-W), or

composition formula: Cu_(1-x)Tl_(x)Ba₂Ca₃Cu₄O_(10-w)

[0049] where 0≦x−1 and 0≦w≦4 is a material having a lowest microwavesurface resistance. Realizing an excellent microwave device makes itnecessary to select a substrate having a low dielectric constant and atthe same time to make a good lattice match between a superconductor thinfilm and the substrate.

[0050] While a single crystal sapphire R (1, −1, 0,2) face substrate isoptimum as it is of low cost, large in surface area and low indielectric constant, it cannot be used as it is because of its too-largelattice mismatch with a Cu family oxide high temperature superconductorthin film.

[0051] In order to solve this problem, it is known to be effective touse a CeO₂ (100) buffer layer on the sapphire R (1, −1, 0,2) facesubstrate.

[0052] A specific example of the CeO₂ layer grown on the sapphiresubstrate is given below.

[0053]FIG. 1 shows an AFM (Atomic Force Microscopic) image of a surfaceof a CeO₂ (100) layer grown on a sapphire R (1, −1, 0, 2) facesubstrate. A specimen was formed from a sapphire R (1, −1, 2, 2) facesubstrate held at a temperature of 525° C. which had a CeO₂ layer builtup thereon to a thickness of 200 nm by magnetron RF sputtering in amixed gas of Ar at 5 mTorr and N₂O at 10 mTorr whereafter it washeat-treated at a temperature of 1100° C.

[0054] As is apparent from FIG. 1, it is seen that CeO₂ grains arerectangular and at the same time are aligned in the directions of <−1,1, 0, 1> and <1, 1, −2, 0>. Thus, CeO₂ is an optimum material to devisea lattice match between the sapphire substrate and the oxide hightemperature superconductor thin film.

[0055] However, if it is attempted to grow an oxide high temperaturesuperconductor thin film epitaxially on a sapphire substrate having CeO₂formed thereon as a buffer layer, the problem arises that the epitaxialgrowth temperature of the oxide high temperature superconductor thinfilm causes Ba in the oxide high temperature superconductor thin film toreact with Ce, forming BaCeO₃, thus preventing the oxide hightemperature superconductor thin film from being satisfactory in bothcrystallographic integrity and crystallographic orientation.

[0056] The present inventors have discovered that this problem is solvedif there is built up on the CeO₂ layer as a first buffer layer a thinfilm as a second buffer layer of an oxide high temperaturesuperconductor in which Ba as a constituent element thereof issubstituted with Sr that does not readily react with Ce and then a Bacontaining oxide high temperature superconductor thin film is allowed togrow epitaxially on the second buffer layer, and have thus arrived atthe present invention. To wit, the thin film in which Sr is substitutedfor Ba in the oxide high temperature superconductor thin film is akin incrystallographic structure and lattice constant to the oxide hightemperature superconductor thin film to be epitaxially grown thereon andextremely high in lattice matching with the latter, and functions as anoptimum buffer layer in which Sr does not readily reacts with Ce; henceit gives the oxide high temperature superconductor thin film excellentcrystallographic integrity and orientation.

[0057] Thus formed in accordance with the present invention, an oxidehigh temperature superconductor in which an oxide high temperaturesuperconductor thin film containing Ba as a constituent element thereofis formed on a crystalline substrate is characterized in that itcomprises a first buffer layer for alleviating or reducing a latticemismatch between the crystalline substrate and the oxide hightemperature superconductor thin film, and a second buffer layer of a Sroxide formed on the first buffer layer in order to act as a barrier toBa diffusion from the high temperature superconductor thin film,interposed between the crystalline substrate and the oxide hightemperature superconductor thin film. As a result, an oxide hightemperature superconductor is obtained that excels in bothcrystallographic integrity and crystallographic orientation by virtue ofthe fact that even if the first buffer layer for reducing the latticemismatch of the substrate with the oxide high temperature superconductorthin film is made of a material that readily brings about a surfacereaction with Ba in the oxide high temperature superconductor thin film,the second buffer layer prevents the occurrence of such a surface orinterfacial reaction.

[0058] Next, a first specific example of the present invention is shownbelow.

[0059] First, mention is made of how a specimen was prepared. A sapphireR (1, −1, 2, 0) face substrate was heat-treated at a temperature of1100° C. for a period of 2 hours, and then its surface was smoothenedand made clean. The resulting sapphire substrate was held at atemperature of 600° C. and had a CeO₂ layer built up thereon to athickness of 15 nm by magnetron RF sputtering in a mixed gas atmosphereof Ar at 5 mTorr and N₂O at 10 mTorr.

[0060] Subsequently, the substrate with its temperature lowered to 50°C. had formed thereon, as the thin film in which Ba in the composition:Cu_(1-x)Tl_(x)Ba₂CaCu₂O_(8-w) is substituted with Sr, an amorphous filmof Cu_(1-x)Tl_(x)Sr₂CaCu₂O_(8-w) to a film thickness of 200 nm bymagnetron RF sputtering.

[0061] Next, this amorphous film had built up thereon an amorphous filmhaving the composition of an oxide high temperature superconductor ofCuTlBa₂Ca₂Cu₃O_(10-w) as to a film thickness of 700 nm by magnetron RFsputtering.

[0062] Thereafter, the specimen was taken out of the magnetron RFsputtering apparatus and was charged, together with a thalliumcontaining high temperature superconductor disk (composed ofCuTl-Ba₂Ca₂Cu₃O_(y) and of 17.5 mm in radius, 4 mm thick and 10 g inweight) and 50 mg of Tl₂O₃ powder particles dispersed thereon, in ahermetically sealed container made of silver (a board shaped containerof 18 mm in radious and 10 mm in height) and was then heat-treated at atemperature of 860° C. for a time period of 30 minutes.

[0063]FIG. 2 is a diagram showing a diffraction pattern by XRD (X-raydiffractometer) of the oxide high temperature superconductor fabricatedin accordance with the present invention. In the diagram, the numeralsaffixed to the peaks of the diffraction pattern indicate theircorresponding Miller face indices, the numerals in the parenthesesindicate their corresponding oxide high temperature superconductors, andthe lettering Al₂O₃ affixed to some peaks indicates that they arediffraction peaks for the sapphire substrate. As is apparent from thediagram, it is seen that a CuTlBa₂Ca₂Cu₃O_(10-w) oxide type hightemperature superconductor fabricated in accordance with a method of thepresent invention, namely CuTl-[1223] is epitaxially grown orientedalong the crystallographic c-axis.

[0064]FIG. 3 is a diagram showing results of measurement by XRDindicating an in-plane orientation of the oxide high temperaturesuperconductor fabricated in accordance with the present invention. Itis a diagram showing results of the measurement by XRD in which with theangle of diffraction 2 θ held at the angle of diffraction for the (107)Miller index face, the specimen is rotated at an angle of rotation φabout an axis perpendicular to the plane of incidence. As is apparentfrom the diagram, it is seen that an oxide high temperaturesuperconductor fabricated in accordance with a method of the presentinvention is excellent in its in-plane orientation, too.

[0065] Shown next are results of experiment confirming that Sr in thesecond buffer layer does not react with Ce in the first buffer layer. Aspecimen used was identical to that used in the first specific exampleexcept that it did not have the amorphous film built up thereon, whichhas the composition of an oxide high temperature superconductor ofCuTlBa₂Ca₂Cu₃O_(10-W), and that it was heat-treated at a temperature of890° C. higher than that in the first specific example. FIG. 4 is adiagram showing a diffraction pattern by XRD indicating that Sr in thesecond buffer layer does not react with Ce in the first buffer layer. Inthe diagram, the numerals affixed to the peaks of the diffractionpattern indicate their corresponding Miller face indices, the numeralsin the parentheses indicate their corresponding oxide high temperaturesuperconductor's type, and the letterings CeO₂ and Al₂O₃ in theparentheses affixed to some peaks indicate that they are diffractionpeaks for the CeO₂ and the sapphire substrate. As is apparent from thediagram, no diffraction peak by SrCeO₃ is observed. It is also shownthat the diffraction strength for CeO₂ remains substantially unchangedcompared with that before the heat-treatment. From these, it has beenconfirmed that no reaction takes place between Sr in the second bufferlayer substituted for Ba in the oxide high temperature superconductorand Ce in the first buffer layer composed of CeO₂.

[0066] Next, a second specific example of the present invention isgiven.

[0067] The second specific example is identical to the first specificexample except that an Ag₂₀ powder is used instead of the Tl₂O₃ powder.

[0068] Using this method, a CuTlBa₂Ca₂Cu₃O_(10-w) oxide type hightemperature superconductor, namely CuTl-[1223], was fabricated. Itsresults of measurement by XRD indicated the same characteristics as inFIGS. 2 and 3, and its superconducting transition temperature Tc andcritical current density Jc were found to be 100 K and 4×10⁴A/cm²,respectively. These superconducting properties are found to be somewhatinferior to those with a CuTl-[1223] oxide type high temperaturesuperconductor fabricated on a SrTiO₃ substrate, but this is apparentlydue to cracking and hence can obviously be improved if a preventivemeasure for the cracking is taken.

[0069] While the present invention has been shown in the above specificexamples as applied to a CuTl-[1223] oxide type high temperaturesuperconductor, it is obvious that the invention is applicable to anyoxide high temperature superconductor containing Ba as a constituentelement and having a composition expressed by

composition formula:Cu_(1-x)M_(x)(Ba_(1-y)Sr_(y))₂(Ca_(1-z)L_(z))_(n−1)(Cu_(1-q)Q_(q))_(n)O_(2n+4-w)or

composition formula:(Cu_(1-x)M_(x))₂(Ba_(1-y)Sr_(y))₂(Ca_(1-z)L_(z))_(n−1)(Cu_(1-q) _(Q)_(q))_(n)O_(2n+4-w)

[0070] where:

[0071] M represents Tl, Hg, Bi, Pb, In, Ga, Al, B, C, Si, Sn, Ag, Au, S,N, P, Mo, Re, Os, Cr, Ti, V, Fe, one element in the lanthanide series,or one or more alkali metal elements,

[0072] L represents Mg, Y, or one or more elements in the lanthanideseries.

[0073] Q represents either or both of Mg and Zn, and

[0074] 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦q≦0.1, 0≦w≦4, and 2≦n≦5.

[0075] While the second buffer layer is shown in the above specificexamples as composed of an oxide high temperature superconductoridentical to a target oxide high temperature superconductor to beepitaxially grown except that Ba therein is substituted with Sr, it isobvious that it may be composed of an oxide high temperaturesuperconductor that is not identical but similar to the target oxidehigh temperature superconductor and which has its Ba constituentsubstituted with Sr.

[0076] While the second buffer layer is shown in the above specificexamples as composed of an oxide high temperature superconductoridentical to a target oxide high temperature superconductor except thatBa therein is substituted with Sr, it is obvious that it may be a Sroxide film that is well lattice-matched with the target oxide hightemperature superconductor.

[0077] Accordingly, as a second form of implementation of the presentinvention, there is provided an oxide high temperature superconductormade by an oxide high temperature superconductor thin film containing Baas a constituent element and formed on a crystalline substrate,characterized in that it comprises a buffer layer composed of a Sr oxideand interposed between the crystalline substrate and the oxide hightemperature superconducting film for reducing a lattice mismatch betweenthem and also for serving as a barrier to the diffusion of Ba from theoxide high temperature superconductor thin film. An oxide hightemperature superconductor that excels in both crystallographicintegrity and orientation can here again be obtained by virtue of thefact that a Sr oxide alleviates or reduces lattice mismatch between thecrystalline substrate and an oxide high temperature superconducting filmand at the same time prevents the interfacial reaction between thecrystalline substrate and Ba in the oxide high temperaturesuperconducting film which will otherwise occur if the substrate is madeof a material that contains a substance liable to interfacially reactingwith Ba from the oxide high temperature superconductor thin film.

[0078] While the present invention has been shown in the above specificexamples as regards a Ba containing oxide high temperaturesuperconductor grown epitaxially on a sapphire substrate, it is obviousthat the invention is not limited to such an oxide high temperaturesuperconductor but is applicable to any one of oxide magnetic material,an oxide dielectric and an oxide conductor, which contains Ba and isepitaxially grown on a sapphire substrate.

INDUSTRIAL APPLICABILITY

[0079] The present invention thus permits fabricating on a substrate oflow dielectric constant an oxide high temperature superconductor that ishigh in crystallographic integrity and at the same time excels incrystallographic orientation.

1. An oxide high temperature superconductor containing Ba as aconstituent element thereof and having a thin film thereof formed on acrystalline substrate, characterized in that it comprises: a firstbuffer layer for reducing a lattice mismatch between said crystallinesubstrate and said oxide high temperature superconductor thin film, anda second buffer layer formed on said first buffer layer and made of anoxide high temperature superconductor that is identical or similar incrystallographic structure and lattice constant to said oxide hightemperature superconductor but in which Ba is substituted with Sr, thesetwo buffer layers interposed between said crystalline substrate and saidoxide high temperature superconductor thin film.
 2. An oxide hightemperature superconductor containing Ba as a constituent elementthereof and having a thin film thereof formed on a crystallinesubstrate, characterized in that it comprises a first buffer layer forreducing a lattice mismatch between said crystalline substrate and saidoxide high temperature superconductor thin film, and a second bufferlayer formed on the said first buffer layer and made of an oxide hightemperature superconductor not containing Ba but containing Sr which issimilar in crystallographic structure and lattice constant to the saidoxide high temperature superconductor, or a Sr oxide which is similar incrystallographic structure and lattice constant to the said oxide hightemperature superconductor, these two buffer layers interposed betweensaid crystalline substrate and said oxide high temperaturesuperconductor thin film.
 3. An oxide high temperature superconductorcontaining Ba as a constituent element thereof and having a thin filmthereof formed on a crystalline substrate, characterized in that itcomprises a buffer layer interposed between said crystalline substrateand said oxide high temperature superconducting film for reducing alattice mismatch between them, wherein said buffer layer reducing thelattice mismatch is made of an oxide high temperature superconductorthat is identical to said oxide high temperature superconductor exceptthat Ba is substituted with Sr therein, or an oxide high temperaturesuperconductor that is identical to said oxide high temperaturesuperconductor except not containing Ba but containing Sr, or a Sr oxidethat reduces the lattice mismatch.
 4. An oxide high temperaturesuperconductor as set forth in any one of claims 1 to 3, characterizedin that said oxide high temperature superconductor containing Ba as aconstituent element thereof consists of a composition expressed bycomposition formula:Cu_(1-x)M_(x)(Ba_(1-y)Sr_(y))₂(Ca_(1-z)L_(z))_(n−1)(Cu_(1-q)Q_(q))_(n)O_(2n+4-w)or composition formula:(Cu_(1-x)M_(x))₂(Ba_(1-y)Sr_(y))₂(Ca_(1-z)L_(z))_(n−1)(Cu_(1-q)Q_(q))_(n)O_(2n+4-w)where: M represents Tl, Hg, Bi, Pb, In, Ga, Al, B, C, Si, Sn, Ag, Au, S,N, P, Mo, Re, Os, Cr, Ti, V, Fe, one element in the lanthanide series,or one or more alkali metal elements, L represents Mg, Y, or one or moreelements in the lanthanide series. Q represents either or both of Mg andZn, and 0≦x≦1,0≦y≦1,0≦z≦1,0≦q≦0.1,0≦w≦4, and 2≦n≦5.
 5. An oxide hightemperature superconductor as set forth in any one of claims 1 to 3,characterized in that said crystalline substrate is a sapphiresubstrate.
 6. An oxide high temperature superconductor as set forth inclaim 5, characterized in that said sapphire substrate has a sapphire Rface (1, −1, 0, 2).
 7. An oxide high temperature superconductor as setforth in claim 1 or claim 2, characterized in that said first bufferlayer is a CeO2 layer.
 8. An oxide high temperature superconductor asset forth in claim 1, characterized in that said second buffer layer hasa composition expressed by composition formula:Cu_(1-x)M_(x)Sr₂CaCu₂O_(8-w) where M represents Ti, Hg, Bi, Pb, In, Ga,Al, B, C, Si, Sn, Ag, Au, S, N, P, Mo, Re, Os, Cr, Ti, V, Fe, an elementin the lanthanide series or one or more alkali metal elements and wherex and w are represented by 0≦x≦1 and 0≦w≦4, respectively.
 9. An oxidehigh temperature superconductor as set forth in claim 1 or claim 2,characterized in that said oxide high temperature superconductor thinfilm is epitaxially grown whereby an amorphous phase composed to formthis oxide high temperature superconductor is deposited on said secondbuffer layer and the deposited amorphous phase is heat-treated in anoxygen atmosphere at a pressure of 1.0 to 10 atm in the presence of Ag₂Oor AgO.
 10. An oxide high temperature superconductor as set forth inclaim 1 or claim 2, characterized in that said oxide high temperaturesuperconductor thin film is epitaxially grown whereby an amorphous phasecomposed to form this oxide high temperature superconductor is depositedon said second buffer layer and the deposited amorphous phase isheat-treated in an oxygen atmosphere at a pressure of 1.0 to 10 atm inthe presence of Tl₂O₃.
 11. An oxide high temperature superconductor asset forth in claim 3, characterized in that said oxide high temperaturesuperconductor thin film is epitaxially grown whereby an amorphous phasecomposed to form this oxide high temperature superconductor is depositedon said buffer layer and the deposited amorphous phase is heat-treatedin an oxygen atmosphere at a pressure of 1.0 to 10 atm in the presenceof Ag₂O or AgO.
 12. An oxide high temperature superconductor as setforth in claim 3, characterized in that said oxide high temperaturesuperconductor thin film is epitaxially grown whereby an amorphous phasecomposed to form this oxide high temperature superconductor is depositedon said buffer layer and the deposited amorphous phase is heat-treatedin an oxygen atmosphere at a pressure of 1.0 to 10 atm in the presenceof Tl₂O₃.
 13. A thin film of an oxide containing Ba as a constituentelement thereof and selected from the group that consists of an oxidemagnetic material, an oxide dielectric and an oxide conductor and formedon a sapphire R (1, −1, 0, 2) face substrate, characterized in that saidthin film has a laminated structure formed by: forming a first bufferlayer made of a CeO₂ thin film on said sapphire substrate, forming onsaid first buffer layer a second buffer layer made of a thin film ofsaid oxide in which Ba is substituted with Sr, and forming said oxide onsaid second buffer layer.
 14. A method of making a thin film of an oxidehigh temperature superconductor containing Ba as a constituent elementthereof by epitaxially growing the oxide high temperature superconductorthin film on a crystalline substrate, characterized in that it comprisesthe steps of: forming on said crystalline substrate a first buffer layeradapted to reduce a lattice mismatch between said crystalline substrateand said oxide high temperature superconductor thin film; forming onsaid first buffer layer a second buffer layer made of an oxide hightemperature superconductor that is identical or similar incrystallographic structure and lattice constant to said oxide hightemperature superconductor but in which Ba is substituted with Sr, anoxide high temperature superconductor not containing Ba but containingSr which is similar in crystallographic structure and lattice constantto said oxide high temperature superconductor, or a Sr oxide which issimilar in crystallographic structure and lattice constant to said oxidehigh temperature superconductor; and epitaxially growing said oxide hightemperature superconductor thin film on said second buffer layer.
 15. Amethod of making a thin film of an oxide high temperature superconductorcontaining Ba as a constituent element thereof by epitaxially growingthe oxide high temperature superconductor thin film on a crystallinesubstrate, characterized in that it comprises the steps of: forming onsaid crystalline substrate a buffer layer adapted to reduce a latticemismatch between said crystalline substrate and said oxide hightemperature superconductor thin film, wherein said buffer layer reducingthe lattice mismatch is made of an oxide high temperature superconductorthat is identical to said oxide high temperature superconductor exceptthat Ba is substituted with Sr therein, or an oxide high temperaturesuperconductor except not containing Ba but containing Sr, or a Sr oxidethat reduces the lattice mismatch; and epitaxially growing said oxidehigh temperature superconductor thin film on said buffer layer.
 16. Amethod of making a thin film of an oxide high temperature superconductoras set forth in claim 14 or claim 15, characterized in that said oxidehigh temperature superconductor containing Ba as a constituent elementthereof consists of a composition expressed by composition formula:Cu_(1-x)M_(x)(Ba_(1-y)Sr_(y))₂(Ca_(1-z)L_(z))_(n−1)(Cu_(1-q)Q_(q))_(n)O_(2n+4-w)or composition formula:(Cu_(1-x)M_(x))2(Ba_(1-y)Sr_(y))₂(Ca_(1-z)L_(z))_(n−1)(Cu_(1-q)Q_(q))_(n)O_(2n+4-w)where: M represents Tl, Hg, Bi, Pb, In, Ga, Al, B, C, Si, Sn, Ag, Au, S,N, P, Mo, Re, Os, Cr, Ti, V, Fe, one element in the lanthanide series,or one or more alkali metal elements, L represents Mg, Y, or one or moreelements in the lanthanide series. Q represents either or both of Mg andZn, and 0≦x≦1,0≦y≦1,0≦z≦1,0≦q≦0.1, 0≦w≦4, and 2≦n≦5.
 17. (Cancelled) 18.A method of making a thin film of an oxide high temperaturesuperconductor as set forth in claim 14 or claim 15, characterized inthat said crystalline substrate is a sapphire substrate.
 19. A method ofmaking a thin film of an oxide high temperature superconductor as setforth in claim as set forth in claim 18, characterized in that saidsapphire substrate has a sapphire R face (1, −1, 0, 2).
 20. A method ofmaking a thin film of an oxide high temperature superconductor as setforth in claim 14, characterized in that said first buffer layer is aCeO₂ layer.
 21. A method of making a thin film of an oxide hightemperature superconductor as set forth in claim 14, characterized inthat said second buffer layer has a composition expressed by compositionformula: Cu_(1-x)M_(x)Sr₂CaCu₂O_(8-w) where M represents Tl, Hg, Bi, Pb,In, Ga, Al, B, C, Si, Sn, Ag, Au, S, N, P, Mo, Re, Os, Cr, Ti, V, Fe, anelement in the lanthanide series or one or more alkali metal elementsand where x and w are represented by 0≦x≦1 and 0≦w≦4, respectively. 22.A method of making a thin film of an oxide high temperaturesuperconductor as set forth in claim 14, characterized in that saidoxide high temperature superconductor thin film containing Ba as aconstituent element thereof is epitaxially grown whereby an amorphousphase composed to form this oxide high temperature superconductor isdeposited on said second buffer layer and the deposited amorphous phaseis heat-treated in an oxygen atmosphere at a pressure of 1.0 to 10 atmin the presence of Ag₂O or AgO.
 23. A method of making a thin film of anoxide high temperature superconductor as set forth in claim 14,characterized in that said oxide high temperature superconductor thinfilm containing Ba as a constituent element thereof is epitaxially grownwhereby an amorphous phase composed to form this oxide high temperaturesuperconductor is deposited on said second buffer layer and thedeposited amorphous phase is heat-treated in an oxygen atmosphere at apressure of 1.0 to 10 atm in the presence of Tl₂O₃.
 24. A method ofmaking a thin film of an oxide high temperature superconductor as setforth in claim 15, characterized in that said oxide high temperaturesuperconductor thin film containing Ba as a constituent element thereofis epitaxially grown whereby an amorphous phase composed to form thisoxide high temperature superconductor is deposited on said buffer layerand the deposited amorphous phase is heat-treated in an oxygenatmosphere at a pressure of 1.0 to 10 atm in the presence of Ag₂O orAgO.
 25. A method of making a thin film of an oxide high temperaturesuperconductor as set forth in claim as set forth in claim 15,characterized in that said oxide high temperature superconductor thinfilm containing Ba as a constituent element thereof is epitaxially grownwhereby an amorphous phase composed to form this oxide high temperaturesuperconductor is deposited on said buffer layer and the depositedamorphous phase is heat-treated in an oxygen atmosphere at a pressure of1.0 to 10 atm in the presence of Tl₂O₃.
 26. A method of making an oxidethin film by epitaxially growing on a sapphire R (1, −1, 0, 2) facesubstrate, an oxide containing Ba as a constituent element thereof andselected from the group that consists of an oxide magnetic material, anoxide dielectric and an oxide conductor, characterized in that themethod comprises the steps of: forming a first buffer layer made of aCeO₂ thin film on said sapphire substrate, forming on said first bufferlayer a second buffer layer made of a thin film of said oxide in whichBa is substituted with Sr, and forming said oxide on said second bufferlayer.